This invention relates generally to non-ferrous metal alloy compositions, and more specifically to wroughtable, nickel alloys which contain significant quantities of chromium and molybdenum, along with the requisite minor elements, to allow successful melting and wrought processing, and which possess high resistance to wet process phosphoric acid and high resistance to chloride-induced localized attack (pitting and crevice corrosion), which is enhanced by deliberate additions of nitrogen.
An important step in the manufacture of fertilizers is the production and concentration of phosphoric acid. This acid is typically made by reacting phosphate rock with sulfuric acid to produce what is often called xe2x80x9cwet processxe2x80x9d phosphoric acid. The resulting xe2x80x9cwet processxe2x80x9d phosphoric acid contains traces of sulfuric acid, along with other impurities from the phosphate rock, such as chlorides, which serve to increase its corrosivity.
To concentrate this xe2x80x9cwet processxe2x80x9d phosphoric acid, several evaporation stages are employed. The evaporator tubes are usually constructed from austenitic stainless steels or nickel-iron alloys, with chromium contents in the approximate range 28 to 30 wt. %, such as G-30 alloy (U.S. Pat. No. 4,410,489), Alloy 31 (U.S. Pat. No. 4,876,065), and Alloy 28. Copper is an essential ingredient in these alloys. These commercial materials possess inadequate resistance to either xe2x80x9cwet processxe2x80x9d phosphoric acid, or chloride-induced localized attack, for use in all evaporation stages, thus necessitating the use of non-metallic materials, with consequent sacrifices in robustness.
Knowing that chromium is beneficial to the xe2x80x9cwet processxe2x80x9d phosphoric acid resistance of austenitic stainless steels and nickel-iron alloys, materials with higher chromium contents have been contemplated. However, thermal stability has been a constraining factor. Simply stated, it is desirable to maintain the face-centered cubic atomic structure in such materials, and excessive alloying results in the formation of deleterious second phases, which impair ductility and corrosion resistance, during wrought processing or welding. Thus, higher chromium levels have hitherto not been feasible in wrought alloys designed for service in xe2x80x9cwet processxe2x80x9d phosphoric acid, given the need to include alloying elements other than chromium, to enhance localized corrosion resistance.
With regard to thermal stability, it is well known that elements such as chromium and molybdenum, which strongly influence resistance to xe2x80x9cwet processxe2x80x9d phosphoric acid and chloride-induced localized attack, are more soluble in nickel than in austenitic stainless steels. It follows that higher levels of alloying are possible in nickel alloys, if iron contents are low. It is not surprising, therefore, that some low-iron nickel alloys exist, with chromium contents in excess of 30 wt. %, and with significant molybdenum additions.
U.S. Pat. No. 5,424,029 discloses such a series of alloys, although these alloys require the addition of tungsten, in the range 1 to 4 wt. %. U.S. Pat. No. 5,424,029 states that such alloys possess superior corrosion resistance to a variety of media, although their resistance to xe2x80x9cwet processxe2x80x9d phosphoric acid is not addressed. Notably, it states that the absence of tungsten results in a significantly higher corrosion rate. This patent does not address nitrogen as an addition.
Another reference which discloses corrosion-resistant nickel alloys with chromium contents in excess of 30 wt. % is U.S. Pat. No. 5,529,642, although the preferred chromium range is 17 to 22 wt. %, and all compositions require the addition of tantalum, in the range 1.1 to 8 wt. %. U.S. Pat. No. 5,529,642 requires a nitrogen addition of between 0.0001 and 0.1 wt. %.
Although all of these prior art alloys are useful corrosion resistant alloys, the levels of copper, tungsten or tantalum reduce thermal stability, and therefore complicate wrought processing and welding. Yet, the prior art deems these elements necessary for optimum corrosion resistance. In fact, copper is regarded as an essential ingredient of G-30 alloy, Alloy 31, and Alloy 28.
Two further U.S. Pat. Nos., 4,778,576 and 4,789,449, disclose nickel alloys with wide-ranging chromium (5 to 30 wt. %) and molybdenum (3 to 25 wt. %) contents, for use as anodes in electrochemical cells. Both patents preferably claim anodes made from C-276 alloy, which contains 16 wt. % chromium and 16 wt. % molybdenum. Nitrogen content is not addressed in these patents. The patents report that electrodes made from this alloy are resistant to corrosion in aqueous alkaline media containing chloride ions and in concentrated hydrochloric acid solutions. But, data reported in U.S. Pat. No. 4,410,489 shows that the alloy does not resist corrosion well in phosphoric acid.
The principal object of this invention is to provide new alloys with higher combined resistance to xe2x80x9cwet processxe2x80x9d phosphoric acid and chloride-induced localized attack than previous alloys, without the need for deliberate additions of tungsten, tantalum, or copper which reduce thermal stability.
It has been found that the above object may be achieved by adding chromium, molybdenum, and requisite minor elements to nickel, within certain preferred ranges. Nitrogen is also a preferred addition, though it is expected that this element will be absorbed into the alloy during air melting. Specifically, the preferred ranges in weight percent are 31.0 to 34.5% chromium, 7.0 to 10.0% molybdenum, up to 0.2% nitrogen, up to 3.0% iron, up to 1.0% manganese, up to 0.4% aluminum, up to 0.75% silicon and up to 0.1% carbon. The most preferred ranges are 32.5 to 34.0% chromium, 7.5 to 8.6% molybdenum, up to 0.15% nitrogen, up to 1.5% iron, 0.1 to 0.4% manganese, 0.2 to 0.4% aluminum, up to 0.5% silicon and up to 0.02% carbon.
It has also been found that these alloys can tolerate impurities that might be encountered from the melting of other corrosion-resistant nickel alloys, especially copper (up to 0.3 wt. %) and tungsten (up to 0.65 wt. %). Up to 5 wt. % cobalt can be used in place of nickel. It is anticipated that small quantities of other impurities, such as niobium, vanadium, and titanium would have little or no effect on the general characteristics of these materials.